Problem: Angie's class has 2 girls for every 3 boys. If there are 20 students in the class, how many girls are in Angie's class?
Solution: If $2k$ is the number of girls in Angie's class, then $3k$ is the number of boys.  The total number of students is $2k+3k=5k$.  Solving $5k=20$ gives $k=4$, so there are $2(4)=\boxed{8}$ girls in Angie's class.